Borehole seismic surveys are conducted for the purpose of mapping subsurface geologic structures. In a typical borehole seismic survey, a source is placed in a borehole at a selected depth and excited in order to produce acoustic waves in the adjacent geological formations. Sensors are usually placed at selected depths in the same or another borehole or at the earth's surface in order to detect the acoustic signals after they have propagated through and been reflected from geological formations.
Borehole source instruments may radiate as tube waves thirty to eighty percent of the energy generated by the source. These tube waves propagate through the fluid which fills the wellbore. The liquid filled borehole is a good conductor of tube waves with the inner surface of the well acting as a wave guide. Relatively strong tube waves travel along the borehole downwardly and upwardly from the borehole seismic source each time it is energized. These tube waves are reflected from the top and bottom of the borehole and from discontinuities in the borehole, but a significant amount of energy in the tube waves are converted into body waves which propagate into the earth subsurface surrounding the well. These secondary sound waves, which travel outwardly from the top and bottom of the liquid columns and other discontinuities, will reach the seismic sensors at a later time that the signals which propagate directly from the source; and these later arrival signals cause the record of the signals from the sensors to be cluttered with unwanted signals and difficult to interpret.
During the seismic surveys, acoustic waves generated by the source which reach the borehole in which the receiver is deployed will also propagate vertically in the receiver borehole in the form of tube waves. These tube waves which propagate within the borehole in which the receiver is placed are also detected by the receiver and interfere with the acoustic signals arriving directly from the borehole source. A problem in borehole seismic surveys is distinguishing the desired acoustic waves detected by the sensors in the borehole from the undesired signals resulting from tube waves.
Seismic processing software endeavors to remove these tube wave produced signals from the signal detected by the receivers; however, if the tube waves can be attenuated before they begin propagating in the borehole, the resulting seismic data will be further improved.
Bob A. Hardage, Crosswell Seismology and Reverse VSP, Vol. 1, Geophysical Press Limited, 1991, pp. 267-269, in a final chapter titled, Future Needs, shows a tube-wave attenuator positioned above and below a source within a wellbore. It was further disclosed on p.147 of Hardage that "tube propagation into a fluid-filled well can be minimized by inserting a large diameter, air-filled canister or bladder in the fluid column (de Bruin, J. A. and Huizer, W., 1989, Radiation from waves in boreholes: Scientific Drilling, 1, 3-10). As stated by Hardage on p. 147-148: "Any downhole source that operates in a liquid-filled well inevitably produces tube waves that propagate away from the source activation point and travel up and down the fluid column. In extremely low velocity formations, these tube waves create conical waves that spread into the surrounding earth. In high velocity formations . . . the majority of the tube wave energy is trapped within the fluid column and the earth disturbance attenuates exponentially as the tube wave signal propagates away from the fluid boundary. In this guided wave mode, large amounts of tube wave energy can be released into the earth at points where there is a significant change in the cross-sectional impedance of the tube wave propagation path. The energy released during one of these tube wave conversions propagates away from the borehole discontinuity in the same manner as would the wavefields produced by point seismic source. "The most desirable downhole sources are those that minimize the amount of energy contained in these fluid-born waves." Hardage also disclosed on page 156 that "interface waves in a fluid-filled well can be virtually eliminated by blocking most of the cross-sectional area of the fluid column with an air-filled bladder or canister.
U.S. Pat. No. 4,858,718, which issued to Chelminski on Aug. 22, 1989 shows a method for attenuating tube waves for use with an impulsive downhole seismic source. The patent discloses the use of an inflatable resilient bladder which may be positioned above and below the seismic source which attenuates the tube waves which emanate from the source. The bladder is inflated in the wellbore, either by gas supplied from a pressurized gas container attached to the attenuator or through a hose line from a source of pressurized gas located on the surface of the Earth near the mouth of the well. A method of dissipating upwardly traveling tube waves is also disclosed in which numerous gas bubbles are released in the upper portion of the liquid column to generate a bubble barrier in the upper portion of the liquid column in the wellbore.
U.S. Pat. No. 5,170,018, which issued to Potier on Dec. 8, 1992 shows the use of absorptive material, such as cork or Sorbothane, deployed in a non-metallic housing above and below a seismic receiver positioned in a borehole, for the purpose of attenuating tube waves.
U.S. Pat. No. 4,817,755, which issued to Gildas discloses a system for downhole seismic signal generation in which cylindrical elements filled with plastic foam material are deployed above and below an impulsive source, such as primacord, to help attenuate the vertical component of the seismic energy and apply the signal substantially to the walls of the borehole.
U.S. Pat. No. 5,171,943, which issued to Balogh et al. on Dec. 15, 1992, discloses a tube wave damper probe for the suppression of borehole tube waves in seismic applications. The damper comprises a gas-filled bladder within a housing. The bladder is filled with gas before the bladder is inserted into the borehole.
U.S. Pat. No. 4,993,001, which issued to Winbow et al. on Feb. 12, 1991, describes an apparatus for converting tube waves to body waves downhole for seismic exploration. A rotary valve tube wave source produces swept frequence tube waves that are injected into a tubing or wellbore. These tube waves are then converted to body waves by an elongate tube wave converter located downhole, comprising an elongate body having a generally cylindrical center section and tapered ends to convert the tube waves to body waves. The tube wave converter is formed from a material, such as metal, which has a strong acoustic impedance contrast with the liquid in the wellbore so as to convert the tube waves into compressional and shear waves that radiate into the earth for use in seismic exploration.
U.S. Pat. No. 5,646,379, which issued to Hsu et al. on Jul. 8, 1997, discloses an attenuator for use in attenuating tube waves in a borehole which includes a body formed from a permeable material having a rigid matrix, such as natural or synthetic rock or a sintered particulate material, which can be saturated with fluid. The shape of the attenuator may be a cylinder or it may be modified to reduce the amount of reflection of tube waves in the borehole. Suggested shapes include two cones placed base to base, or a cylinder having conically tapered ends.
S. T. Chen, "A Single-Well Profiling Tool and Tube Wave Suppression" Expanded Abstract, SEG, 13-16, 1993, discusses an acoustic source comprising a stack of hollow PZT cylinders driving two end hemispherical masses.
W. T. Balogh, "The Borehole Tubewave Damper Probe" Expanded Abstracts, SEG, 159-162, 1992, describes a tube wave damper probe which utilizes a gas-filled bladder to attenuate borehole tube waves.
L. D. Pham, C. E. Krohn, T. J. Murray, and S. T. Chen, "A Tube Wave Suppression Device for Cross-Well Applications" Expanded Abstracts, SEG, 17-20, 1993 describes the use of a cylindrical porous but impermeable body between the source and receiver. Mineral which is proposed for such use is a material called "syntactic foam". UK Patent Application GB 2253054 describes the use of cork, an absorptive, compressible material, in the form of disks and balls as an attenuator. However, the effectiveness of both cork and syntactic foam is depth dependent.
U.S. Pat. No. 5,005,666, which issued to Fairborn on Apr. 9, 1991, discloses the use of inflatable bladders with seismic receivers in a borehole in order to isolate the receivers from tube waves. However, there are no teachings regarding what the bladders should be made of, what they should be inflated with, or when they should be inflated.